Not Applicable
Not Applicable
1. Field of the Invention
This invention relates to an apparatus for preventing cross-filtration of relatively cool and warm air masses at the opening of a refrigerated space, and particularly, for an air curtain that continuously monitors and optimizes the condition of the circulating air stream to eliminate the formation of fog and frost at the opening.
2. Background and Description of the Prior Art
Refrigerated warehouses typically have one or more cold storage rooms adjacent to rooms at more moderate temperatures. At open doorways between these rooms some of the lighter, warm air will flow into the cool area primarily through the top of the doorway (warm air infiltration) in exchange for heavier, cool air at the bottom of the doorway (cold air exfiltration). Depending upon the conditions of the two air masses, this cold air exfiltration and warm air infiltration can cause numerous problems. Infiltrating warm air can carry more moisture than cool air and tends to become supersaturated within the cold room or at the door opening, which leads to precipitation or airborne ice crystals at the doorway. The humid warm air also leads to ice build-up within the cold room, especially on the floor, doors, walls, evaporator coils, and/or products adjacent to the doorway and inflates energy costs for refrigerating the cold rooms. The exfiltrating cool air tends to mix with the humid warm air to cause fog at the warmer side of the doorway. The fog reduces visibility and can lead to wet slippery floors at the doorway.
There have been many attempts in the prior art to minimize the adverse effects of the colliding air masses at the doorways of cold storage rooms. Commonly, a physical barrier of some type is utilized at the doorway. Such barriers include sliding doors having overlapping edges or sweeps that reduce the air flow through the gaps around the door panels. Sliding doors hamper passage through the doorway and allow heavy infiltration during high door operation periods. These high door operation periods can cause ice build-up and mass air infiltration and exfiltration. Another type of physical barrier is a strip door with transparent plastic or vinyl strips hanging from the doorway header. Strip doors are typically low-cost but they impede passage through the doorway, and the strips can separate with use allowing cross-filtration of the air. Once this begins to occur, the strips can become coated with ice so as to reduce visibility through the doorway and allow for greater air infiltration.
Rather than a physical barrier, the doorways of cold storage rooms can be xe2x80x9cclosedxe2x80x9d by an active air barrier, commonly referred to as an xe2x80x9cair curtainxe2x80x9d, allowing unobstructed passage through the doorway. An air curtain is formed by a fan or air mover producing a relatively high velocity air stream across the doorway, either from side to side or top to bottom, to counteract the forces of the cross-filtrating air masses. A heater may be used to condition the air stream so as to maintain the temperature of the air sufficient to prevent saturation (and thus condensation) of the air at the doorway.
One known air curtain apparatus is disclosed in U.S. Pat. No. 4,516,482. This patent discloses an air curtain vestibule mounted at a freezer door having duct-work containing an air mover and a heater. The apparatus is designed to heat the air curtain to a temperature sufficient to avoid super-saturation of air infiltrating the freezer and saturation of exfiltration air entering the outside of the freezer (the warmer side). This is accomplished by heating the recirculating air to a temperature that brings the mixed air to a point along a line tangent to the saturation curve (100% humidity line) on the psychrometric chart. This allows for air infiltration and exfiltration without condensation. The recirculating air temperature is determined based on normal operating conditions of the freezer and the anteroom (outside the freezer) and the heater is set to maintain this temperature. U.S. Pat. No. 6,106,387 discloses a similar air curtain, however, using an electronic controller and a plurality of temperature and humidity sensors. Like the ""482 patent, the heater is operated to maintain a pre-selected temperature that results in a relative humidity along a line tangent to the saturation curve on the psychrometric chart.
Since these systems maintain a pre-selected temperature, they do not adjust operating parameters in response to transient or changed conditions of the air masses, for example, due to significant changes in weather or anteroom conditions. As such, the system can operate improperly (causing fog or frost) and in an energy inefficient manner until the condition returns to normal or until the improper condition is detected and the system is manually reconfigured to adjust operating parameters, which may require one or more time consuming and costly service calls.
The present invention improves upon prior art air curtains by continuously monitoring the state, for example the temperature and humidity, of one or both ambient air masses and the curtain air stream. The air stream is then conditioned in a way that minimizes air cross-filtration and energy consumption while maintaining the temperature and humidity of the air stream at a point substantially along or slightly below a line representing the mixing of the air masses and the air stream that is tangent to the psychrometric saturation curve.
In particular, the present invention is an apparatus for forming an air stream across a doorway between relatively cool and warm air areas. The apparatus includes an air mover for moving an air stream across the doorway and a heater in thermal communication with the air stream for warming the air stream. The apparatus also includes an electronic control unit controlling the operation of the heater as well as at least one air sensor located in at least one of the air areas providing an air input to the control unit and a second air sensor located in communication with the air stream providing an air stream input to the control unit. The control unit continuously monitors the air and air stream inputs and operates the heater to maintain the temperature of the air stream at a point substantially along a line representing the mixing of air from one or both of the relatively warm and cool air areas and the air stream that is tangent to the psychrometric saturation curve.
The invention also provides a method of maintaining a non-saturated air curtain across a doorway between relatively cool and warm air areas so as to prevent condensation and the formation fog and frost at the doorway. The method includes monitoring continuously the condition of the relatively warm and/or cool ambient air areas and the condition of the air stream and conditioning the air stream to maintain its temperature and humidity at a point substantially along or slightly below a line that is tangent to the psychrometric saturation curve representing the mixing of the air and the air stream.
By continuously monitoring the state of the air stream and at least one of the air masses, the apparatus of the present invention can efficiently prevent the air stream from becoming saturated and forming condensation at the doorway. The heater is operated by the controller in real-time to maintain the temperature and relative humidity of the air stream just below saturation. The controller approximates the psychrometric curve using a unique quadratic equation and computes the necessary air stream temperature by evaluating the mixing line equation with the values input by the temperature and humidity sensors. This temperature also corresponds to the lowest approximate air stream temperature maintaining non-saturation, thereby minimizing heat input (and associated costs) and improving energy efficiency of the system.
In a preferred form, the apparatus is an air curtain having a supply air plenum with an outlet aperture at a first side of the doorway, a return air plenum with an inlet aperture at a second side of the doorway and an intermediate air plenum extending between the supply and return air plenums. The air curtain includes pairs of temperature and humidity sensors, one pair preferably located in the relatively warm air area and the other pair located in the air stream. The temperature and humidity sensors provide signals to the control unit indicating the temperature and humidity at the warm area and the air stream, which are processed by the control unit for operation of the heater.
In other preferred forms, the air curtain further includes pressure sensors located in the relatively cool and warm air areas providing respective cool and warm air pressure input signals to the control unit. The control unit continuously monitors the pressure input and operates the air mover at a threshold pressure differential to minimize cross-filtration through the doorway. The air curtain can also include an air speed sensor detecting air velocity through the doorway and providing an air cross-filtration speed input to the control unit. The control unit continuously monitors the air cross-filtration speed input signal and operates the air mover to minimize cross-filtration through the doorway. Still further, the air curtain can be designed to mix dehumidified air flow with the air stream, for example, air from the cool air area.
The air curtain thus substantially reduces cross-filtration, and the related adverse effects, of two or more adjacent air masses at the opening of a cooled space. In particular, the air curtain substantially reduces warm air infiltration and cold air exfiltration. By continuously monitoring pressure sensors located in the relatively cool and warm air areas and/or the velocity of air passing through the doorway, the air mover can be operated in real-time to adjust the air curtain speed as needed due to random or other changes in the state of the air masses. The air curtain of the present invention also reduces or eliminates air stream saturation and condensation by mixing dehumidified or low moisture air with the air stream.
The foregoing and other objects and advantages of the invention will appear from the following description. In this description reference is made to the accompanying drawings which form a part hereof and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made therefore to the claims for interpreting the scope of the invention.